For digital systems, accurate timing is crucial to data transmission. Clock signals set the timing for the components in a digital system and therefore are crucial to proper operation. A computer motherboard, for example, will transmit a master clock signal to integrated circuit boards, chipsets, peripherals, microprocessors, or other components connected to the motherboard. In this way, system components may be synchronized together using a shared clock signal.
Various techniques exist for generating and distributing clock signals within a digital system. For example, a primary clock signal might be generated by a ring oscillator or separate clock chip using a crystal oscillator and then routed from the generator to the devices connected to the clock. These techniques involve electrical clock signals, i.e., clock signals traveling along metallic or semiconductor conduits. Unfortunately, electrical clock signals present numerous design limitations.
Ideally, clock signals would have a well defined duration, consistent shape, and zero propagation path dependence. In reality, electrical clock signals have variable rise and fall times, noticeable jitter and a path-dependent skew that arises from timing differences and waveform variations between clock signals. Electrical clock signals also have limited bandwidth.
Typically, clock signals are distributed throughout a system via a distribution network. In theory, the network would make duplicate copies of a clock signal and provide identical paths for each duplicate copy. In reality, however, skew problems abound, primarily due to electrical load differences among the various paths and parasitic effects within the network.
Recently, some have proposed moving away from a purely electrical digital clocking system to an optical clocking system. Using optical signals, i.e., light pulses, presents some obvious theoretical advantages. Optical signals are not susceptible to load variations or parasitic effects because they travel through waveguides and not conducting metallic wires. Also, optical signals may transmit at much faster clock rates, allowing for THz range clock cycles, while electrical clock signals have a theoretical limit of about 25 GHz for 5 mm transmission distances. Thus, optical clock signals can provide orders of magnitude faster performance capabilities.
In the optical networks proposed for clock signal distribution, a network distributor generates or receives a clock signal, and that signal is then split into multiple signals by either a simple Y branch splitter or a multimode interferometer. Each copy of the clock signal is then provided to an output waveguide. In some devices, an optical H-tree structure has been proposed. An optical H-tree has three Y branches that form an H-shaped layout with an input at the center of the H-tree structure.
While optical networks do not have the impedance load variation and parasitic problems of electrical domain networks, they have their share of shortcomings. One of the main problems affecting optical networks is modal confinement. To maintain its waveform and intensity, a signal's mode must be confined to the propagating waveguide. This means that only straight waveguides or waveguides of certain, typically large, bending radii have been proposed. The bending radii are determined by the index contrast of the waveguide core and its cladding layer. A large bending radius is used to avoid signal loss. Unfortunately, these limitations result in large devices of limited scalability. The problem is multiplied with network complexity.